Xeroderma Pigmentosum is a well established model for studying the molecular basis of skin cancer. All XP patients are predisposed to the development of actinic keratosis and a variety of skin tumors on sun exposed areas of the body. The disease is complex, represented by at least 8 genetic complementation groups, 7 of which have been associated with defective nucleotide excision repair (NER), the pathway whereby normal cells remove bulky base damage from DNA. Despite this significant progress, the second largest XP genetic complementation group (the XP variant, or XP-V) is not well understood. The cells from these patients are not defective in NER, yet the patients exhibit the same photosensitivity and predisposition to skin cancer seen in the classic NER-defective patients. While UV radiation produces a delay in DNA synthesis in both normal and XP-V cells, several lines of evidence suggest that this delay is exaggerated in XP-V and may represent a defect in an inducible replication bypass mechanism. This observation, in conjunction with cell fusion techniques, offers a quantifiable assay that could be used i) to determine if all XP-V patients represent the same genetic defect and ii) to map the normal gene which complements this abnormal response. By labeling the membranes of XP-V and normal cells with different fluorescent markers, it will be possible to identify whole cell fusion products and evaluate correction of the defective XP-V response in those heterokaryons following UV radiation. Although fusion to normal cells is expected to complement the XP-V defect, fusion between different XP-V cell lines will not render correction if all XP-V patients represent mutations in the same gene. Following these whole cell fusion experiments, microcell mediated chromosome transfer techniques will be used to transfer only one or a few normal human chromosomes into XP-V cells. Fluorescence in situ hybridization with chromosome-specific probes will be used to correlate the presence or absence of each normal chromosome with complementation, facilitating mapping of the complementing locus. This mapping should yield candidate genes which have already been mapped to this chromosome of interest by others for further evaluation.